Proton exchange membrane fuel cells (PEMFC) are electrochemical energy converters, which directly transform chemical energy into electricity. They are an alternative to power generators, due to numerous benefits such as lower emissions, high efficiencies, and lower maintenance requirements (Nature 414, 2001, 332). In typical PEMFC design, a proton-conducting polymer membrane (also referred to as electrolyte membrane and polymer electrolyte membrane) is disposed between two electrodes, namely a cathode electrode and an anode electrode.
The proton exchange membrane is a vital part in the design of improved polymer electrolyte membrane fuel cells. It has three main functions: as an electrolyte medium for ion conduction and electrode reactions, as a barrier for separating reactant gases, and as a support for electrode catalysts. The membrane must not only be a good conductor for hydrogen ions, but also an electronic insulator; it must have low permeabilities to the reactant fuel and oxidant (hydrogen or methanol and oxygen); it must show mechanical strength; and it must be chemically and thermally stable under the cell operation conditions (Chem. Rev. 104, 2004, 4587).
A variety of proton exchange membranes have been developed over the years. However, to date, these membranes do not possess the combination of properties required for operation at elevated temperatures.
Technologies based on perfluorinated polymers with sulfonic acid groups in side chains, such as Nafion® PFSA polymers (Chem. Rev. 104, 2004, 4535), demonstrate excellent chemical stability. However, such membranes are not suitable at high temperatures or under low relative humidity conditions. Moreover, these membranes exhibit methanol crossover, which results in poor efficiency and limited performance levels. These membranes are costly.
Sulfonated polymer materials have been developed as alternatives to PFSA (J. Membr. Sci. 185, 2001, 3, Annu. Rev. Mater. Res. 33, 2003, 503, Chem. Mater. 15, 2003, 4896). The most widely investigated systems include sulfonation of polysulfones (J. Polym. Sci., Part A, 34, 1996, 2421, J. Membr. Sci., 139, 1998, 211, J. Appl. Polym. Sci. 77, 2000, 1250), polyetheretherketones (J. New Mater. Electrochem. Syst., 3, 2000, 93, Macromolecules, 25, 1992, 6495, Solid State Ionics, 106, 1998, 219), polyetherketoneketones (Solid State Ionics 125, 1999, 213), polyimides (J. Power Sources, 160, 1999, 127, Macromolecules, 35, 2002, 6707, J. New Mater. Electrochem. Syst., 4, 2001, 115), rigid rod poly(p-phenylene) (Macromolecules, 27, 1994, 1975, Macromol. Rapid Commun., 15, 1994, 669) polyphenyleneoxides (J. Appl. Polm. Sci., 36, 1998, 1197), polythiophenylenes (Macromolecules, 30, 1997, 2941), and polyphenylquinoxalines (J. Polym. Sci., 36, 1998, 1197).
However, PFSA and sulfonated aromatic polymer materials limit the cell operation below 90° C. At such temperatures, the presence of impurities such as carbon monoxide in the hydrogen feed will have a poisonous effect on the electrocatalyst. Electrocatalysts have also been developed for a typical operational temperature of 80° C., but 50-100 ppm of carbon monoxide can deactivate these catalysts. The need for humidified gases as well as the demand high purity hydrogen result in increased operation costs.
Recently polybenzimidazole (PBI) materials have been proposed for use in proton exchange membranes. U.S. Pat. No. 5,091,087, for example, relates to a process for preparing a microporous PBI membrane. Phosphoric acid-doped polybenzimidazole (PBI), in particular, has emerged as a promising candidate for a low cost and high performance fuel cell membrane material. Apart from high thermal stability and good membrane-forming properties, PBI contains basic functional groups which have been shown to easily interact with strong acids (Solid State Ionics, 118, 1999, 287, Electrochim. Acta 45, 2000, 1395, J. Electrochem. Soc., 151(1), 2004, A8) such as H3PO4 and H2SO4, thereby allowing proton migration along the anionic chains. PBI membranes further provide high glass transition temperature (above 400° C.) and the ability of impregnated acid to act as a proton solvating species, thereby making them proton conductive. PBI membranes also show high proton conductivity at high temperature (>100° C.) under low relative humidity conditions, and have a high CO tolerance. PBI has also been shown to become a proton conductor at temperatures up to 200° C. when sulfonated (U.S. Pat. No. 4,814,399), phosphonated (U.S. Pat. No. 5,599,639), or doped with a strong acid (U.S. Pat. No. 5,525,436 and J. Electrochem. Soc. 142, 1995, L21). Such polymer membranes can be used as an electrolyte for PEM fuel cells with various types of fuels such as hydrogen (Electrochim. Acta, 41, 1996, 193), methanol (J. Appl. Electrochem. 26, 1996, 751), trimethoxymethane (Electrochim. Acta, 43, 1998, 3821) and formic acid (J. Electrochem. Soc. 143, 1996, L158). PBI also exhibits high electrical conductivity (J. Electrochem. Soc. 142, 1995, L21), low methanol crossover rate (J. Electrochem. Soc. 143, 1996, 1225), nearly zero water drug coefficient (J. Electrochem. Soc. 143, 1996, 1260), and enhanced activity for oxygen reduction (J. Electrchem. Soc. 144, 1997, 2973). However, the availability of PBI is limited. Further, PBI presents only moderate mechanical properties and has low oxidative stability.
Another recent approach is the use of ionically cross-linked acid-base blends that posses high conductivity, thermal stability, and mechanical flexibility and strength. Combination of acidic polymers (sulfonated polysulfone, sulfonated polyethersulfone or sulfonated polyetheretherketone) and basic polymers (polybenzimidazole (PBI), polyethyleneimine and poly(4-vinylpyridine)) have been explored (Solid State Ionics 125, 1999, 243, J. New Mater. Electrochem. Syst. 3, 2000, 229). Sulfonated polysulfone/PBI membranes doped with phosphoric acid have also been investigated exhibiting chemical and thermal stability and good proton conductivity (Macromolecules 33, 2000, 7609, Electrochim. Acta 46, 2001, 2401, J. Electrochem. Soc. 148, 2001, A513). Blends of PBI with aromatic polyether copolymer containing pyridine units in the main chain have also been prepared, resulting in easily doped membranes with good mechanical properties and oxidative stability (Journal of the Membrane Science 2003, 252, 115).
Polymeric materials have recently been under development which aim to fulfill the prerequisites for use in high temperature PEMFCs. Poly(2,5-benzimidazole) (ABPBI) is the simplest polybenzimidazole type polymer (Fuel Cells 5, No3, 2005, 336, Electrochem. Commun. 5, 2003, 967, J. Membr. Sci. 241 2004, 89, Macromol. Rapid Commun. 25, 2004, 894) with thermal stability and conducting properties as good as those of PBI, and with comparable performance in high temperatures PEM fuel cells. High-temperature aromatic polyether type copolymers (Macromol. Rapid Commun. 26, 2005, 1724, J. Membr. Sci. 252, 2005, 115, J. Membr. Sci. 197, 2002, 231, J. Polym. Sci.: Part A: Polym. Chem., 41, 2003, 2264) containing basic groups like PBI have been shown to enable formation of complexes with stable acids and to exhibit high thermal and chemical stability, and good conducting properties which are useful in high temperature PEMFCs. However, such materials still do not possess the combination of properties required for use in high temperature fuel cells.
Some recent technologies are directed to the direct catalyzation of membranes, in particular aqueous based polymer electrolytes such as perfluorinated sulfonic acids. These approaches, however, are limited by the ability to translate developments to mass manufacturability while maintaining reproducibility (batch vs. continuous) and while keeping costs reasonable. Depending on the deposition methods used, these approaches can be classified into the following broad categories, (i) thin film formation with carbon supported electrocatalysts, (ii) pulse electrodeposition of noble metals (Pt and Pt alloys), (iii) sputter deposition, (iv) pulse laser deposition, and (v) ion-beam deposition. While all of these approaches aim to improve the charge transfer efficiency at the interface, some of the approaches provide for a better interfacial contact which allows for efficient movement of ions, electrons, and dissolved reactants in the reaction zone, while other approaches additionally effect modification of the electrocatalyst surface (such as those rendered via sputtering, electrodeposition or other deposition methods).
Several variations of first approach, which uses thin film technology in conjunction with conventional carbon supported electrocatalysts, have been reported. One such approach is the so called “decal” approach, wherein the electrocatalyst layer is cast on a PTFE blank and then decaled on to the membrane (J. App. Electrochem. 22, 1992, 1; J. Power Sources 71, 1998, 174). Alternatively an “ink” comprising of Nafion® solution, water, glycerol and electrocatalyst is coated directly on to the membrane (in the Na+ form) (J. Electrochem. Soc. 139(2), 1992, L28). These catalyst coated membranes are subsequently dried, typically under vacuum, 160° C., and ion exchanged to the H+ form (J. App. Electrochem. 22, 1992, 1). Modifications to this approach have been reported with variations in choice of solvents and heat treatment (J. Power Sources 113(1), 2003, 37; Electrochemica Acta 50(16-17), 2005, 3200) as well as choice of carbon supports with different microstructure (J. Electrochem. Soc. 145(11), 1998, 3708). Other variations to the “thin film” approach have also been reported such as those using variations in ionomer blends (WO Pat., (E.I. Dupont de Nemours and Company, USA). 2005, 24 pp.), ink formulations (GS News Technical Report 63(1), 2004, 23), spraying techniques (Proc.-Electrochem. Soc. 94-23 (Electrode Materials and Processes for Energy Conversion and Storage), 1994, 179; IN Pat., (India). 1998, 13 pp), pore forming agents (Dianhuaxue 6(3), 2000, 317), and various ion exchange processes (GS News Technical Report 62(1), 2003, 21). At its core, this approach relies on extending the reaction zone further into the electrode structure away from the membrane, thereby providing for a more three dimensional zone for charge transfer. Most of the variations reported above enable improved transport of ions, electrons, and dissolved reactant and products in the “reaction layer” motivated by need to improve electrocatalyst utilization. These attempts, in conjunction with use of Pt alloy electrocatalysts, have formed the bulk of the current state of the art in the PEM fuel cell technology. The limitations of this approach include problems with controlling the Pt particle size (with loading on carbon in excess of 40%), problems with uniformity of deposition in large scale production, and cost (due to several complex processes and/or steps involved).
An alternative method for enabling higher electrocatalyst utilization has been attempted with pulse electrodeposition. For example, Taylor et al. (J. Electrochem. Soc. 139(5), 1992, L45) is one of the first to report this approach. In accordance with Taylor's process, pulse electrodeposition with Pt salt solutions, which relied on their diffusion through thin Nafion® films on a carbon support, provides electrodeposition in regions of ionic and electronic contact on the electrode surface. A recent review on this method by Taylor et al., describes various approaches to pulse electrodeposition of catalytic metals (U.S. Pat. No. 6,080,504). In principal, this methodology is similar to the thin film approach described above, albeit with a more efficient electrocatalyst utilization, since the deposition of electrocatalysts theoretically happens at the most efficient contact zones for ionic and electronic pathways. Improvements to this approach have been reported by Antoine and Durand (Electrochem. and Solid-State Lett. 4(5), 2001, A55) and by Popov et al., (Plating and Surface Finishing 91(10), 2004, 40). Developments in the pulse algorithms and cell design have enabled narrow particle size range (2-4 nm) with high efficiency factors and mass activities for oxygen reduction. Though these methods provide additional benefits, there are concerns on the scalability of such methods for mass scale manufacturing.
Sputter deposition of metals on carbon gas diffusion media is another alternative approach. However, the interfacial reaction zone is more in the front surface of the electrode at the interface with the membrane. The original approach involved applying a layer of sputter deposit on top of a regular Pt/C containing conventional gas diffusion electrode. Such an approach exhibited a boost in performance by moving part of the interfacial reaction zone in the immediate vicinity of the membrane (Electrochemica. Acta 38(12), 1993, 1661). Recently, Hirano et al. (Electrochim. Acta 42(10), 1997, 1587) reported using a thin layer of sputter deposited Pt on wet proofed non-catalyzed gas diffusion electrode (equivalent to 0.01 mgpt/cm2) with similar results as compared to a conventional Pt/C (0.4 mgpt/cm2) electrode obtained commercially. Cha and Lee (J. Electrochem. Soc. 146, 1999, 4055), report an approach with multiple sputtered layers (5 nm layers) of Pt interspersed with Nafion®-carbon-isopropanol ink, (total loading equivalent of 0.043 mgpt/cm2) which exhibited equivalent performance to conventional commercial electrodes with 0.4 mgpt/cm2. Huag et al. (J. Electrochem. Soc. 149, 2002, A862) studied the effect of the substrate on the sputtered electrodes. Further, O'Hare et al., studied the sputter layer thickness, and reported that best results were obtainable with a 10 nm thick layer. Further, advancements have been made with sputter deposition as applied to direct methanol fuel cells (DMFC) by Witham et al. (Electrochem. and Solid-State Lett. 3(11), 2000, 497; Proc.-Electrochem. Soc. 2001-4 (Direct Methanol Fuel Cells): 2001, 114), wherein several fold enhancements in DMFC performance were reported compared to electrodes containing unsupported PtRu catalyst. Catalyst utilization of 2300 mW/mg at a current density of 260 to 380 mA/cm2 was reported (Electrochem. and Solid-State Lett. 3(11), 2000, 497; Proc.-Electrochem. Soc. 2001-4 (Direct Methanol Fuel Cells): 2001, 114). While the sputtering technique provides for a cheaper direct deposition method, the principal drawback is durability. In most cases, the deposition has relatively poor adherence to the substrate, and under variable conditions of load and temperature, there is a greater probability of dissolution and sintering of the deposits.
An alternative method dealing direct deposition was recently reported using pulsed laser deposition (Electrochem. and Solid-State Lett. 6(7), 2003, A125). Excellent performance was reported with loadings of 0.017 mgpt/cm2 in a PEMFC. However this was only applicable with the anode electrodes, and no cathode application has been reported to date.
All these new direct deposition methodologies provide questionable mass manufacturability with adequate control on reproducibility. 3 M company has proposed approaches for mass manufacture of electrodes with low noble metal loading (U.S. Pat. No. 5,879,828). In accordance with these approaches, a series of vacuum deposition steps are used with appropriate selection of solvents and carbon blacks, which results in nanostructured noble metal containing carbon fibrils which are embedded into the ionomer-membrane interface (U.S. Pat. No. 5,910,378 and U.S. Pat. No. 5,879,827).
Another alternative technique involves the use of ion-beam techniques, wherein low energy ion bombardment concurrent to thin film vacuum deposition (electron beam) is used to provide dense, adhering, and robust depositions. Mechanisms of ion/solid interactions during thin film growth as well as development of various protocols for specific application areas (including tribology, anti corrosion coatings, superconducting buffer layers and coatings on temperature sensitive substrates such as polymers) have been studied (Mat Res. Soc. Symposium Proceedings 792(Radiation Effects and Ion-Beam Processing of Materials): 2004, 647). Modifications of this approach to prepare 3-D structures including overhang and hollow structures have also been recently reported (J. Vac. Sci. Tech., B: Microelectronics and Nanometer Structures-Processing, Measurement, and Phenomena 21(6), 2003, 2732). Use of dual anode ion source for high current ion beam applications has also been reported recently (Rev. Sci. Inst. 75(5, Pt. 2), 2004, 1934), where benefits for mass production environment are discussed.